Further, in the Political Schematics of the Medieval World, it was run under the principle of the "Respublica Christiana", where all Kings were vassals of the Pope, and no one was truly independent. According to this, under the Medieval Law, since William I was not a direct vassal of the Pope, but of the King of France, then any and all of his lands were in theory propriety of the King of France. The King could in theory revoke William from his fiefs, which included obviously England. Just like Massilia was a part of the Roman Republic before its annexation or much of the British African Empire was part of Africa. Those were protectorates, where the protected entity payed tribute and fealty to its Lord. Massilia -> Rome ; Bechualand -> Britain ; William -> King of France. Of course this is all theory. But de jure, England was part of France, and since I'm not sure if any French King ever withdrew his claims to England, in which case the French Republic, as the lawful successor of the French Kingdom, is still entitled to all claims which the kings of old did not relenquish.